1. Field of the Invention
The present invention relates to a nonreducing dielectric ceramic composition and, more particularly, to a nonreducing dielectric ceramic composition used, for example, in a monolithic ceramic capacitor and the like.
2. Description of the Prior Art
For manufacturing monolithic ceramic capacitors, a dielectric material coated with a sheet of an electrode material is first prepared. The electrode material serves as an internal electrode. A material having BaTiO.sub.3 as the main component is used as the dielectric material. Dielectric ceramics having an internal electrode are obtained by pressing stacked dielectric materials coated with the electrode material into a monolithic multi-layer body, which is fired in a natural atmosphere at 1250.degree. to 1350.degree. C. Then, an external electrode conducting with the internal electrode is baked on edge faces of the dielectric ceramic to obtain the monolithic ceramic capacitor.
With regard to the material of the internal electrode, the following conditions must be satisfied: (a) since the dielectric ceramics and the internal electrode are baked simultaneously, the melting point must be higher than the firing temperature of the dielectric material; and (b) the material must not oxidize even in an oxidizing atmosphere, and the material must not react with the dielectric material.
An electode material which satisfies these conditions is the noble metals, such as platinum, gold and palladium.
However, while these electrode materials have superior characteristics, they are very expensive. Therefore, the cost of the electrode material in the monolithic ceramic capacitor is 30 to 70% of the whole cost of the capacitor, and is the biggest factor in the manufacturing cost of a monolithic ceramic capacitor.
There are base metals, such as Ni, Fe, Co, W and Mo, besides the nobel metals which have a high melting point. These base metals however, are liable to oxidize in an oxidizing atmosphere and not function as the electrode. Accordingly, in order to use these base metals as the internal electrode of a monolithic ceramic capacitor, they must be fired in a neutral or reducing atmosphere together with the dielectric material. The conventional dielectric ceramic materials, however, when fired in a reducing atmosphere, are reduced remarkably and become semiconductive.
In order to avoid the problems discussed above, a dielectric material having a ratio of Ba sites/Ti sites above the stoichiometric ratio in a barium titanate solid solution was devised. See, for example, Japanese Patent Publication No. 42588/82 and U.S. Pat. No. 4,115,493. By using such a dielectric material, a dielectric ceramic is obtained which does not become semiconductive even when fired in a reducing atmosphere, enabling manufacture of a monolithic ceramic capacitor using a base metal, such as nickel and the like, as the internal electrode.
Miniaturization of electronic components in the development of electronics has progressed rapidly in recent years resulting in a trend to miniaturize monolithic ceramic capacitors.
In the miniaturizing of monolithic ceramic capacitors, a dielectric layer is formed into a thin film. When the thickness of the thin film is below 10 m, the number of crystal grains is reduced in one layer of ceramic sheet, so that the reliability of the monolithic ceramic capacitor is reduced.